1. Field of the Invention
The invention relates to an FM quadrature demodulator having in-phase and quadrature terminals for applying a pair of FM-modulated signals in a mutual phase quadrature thereto, comprising a modulation signal which is frequency-modulated on a carrier, one of the two terminals being coupled to a first input of a first phase comparison circuit and the other terminal being coupled to a second input of the first phase comparison circuit via a first phase-shifting circuit which realises a phase shift varying with said modulation signal, said first phase comparison circuit being connected to an output of the FM quadrature demodulator via a low-pass filter.
2. Description of the Related Art
An FM quadrature demodulator of this type is known per se, for example from German Patent Application no. 26 36 268.
The in-phase terminal of the known FM quadrature demodulator functions as a demodulator input. Via this in-phase terminal, an FM-modulated input signal is applied to a first-order differential network preceding a limiter, as well as to a first phase-shifting circuit.
The first-order differential network is used for a more or less frequency-independent 90.degree. phase shift of the FM-modulated input signal, but also realizes a signal gain which increases with the frequency. This frequency-dependent signal gain is eliminated by means of the limiter, so that an FM-modulated signal, which is in phase quadrature with respect to the FM-modulated input signal, is obtained at the output of the limiter. The FM-modulated quadrature signal is applied to the first input of the first phase comparison circuit via the output of the limiter which functions as a quadrature terminal of the known FM quadrature demodulator.
The first phase-shifting circuit of the known FM quadrature demodulator realizes a frequency-dependent phase shift of the FM-modulated input signal applied to the input terminal, or in-phase terminal, with an average phase shift of zero degrees at the frequency of said carrier. The output signal of the first phase-shifting circuit is subsequently applied to the second input of the first phase comparison circuit. The two FM-modulated signals applied to the first and second inputs are multiplied by each other in the first phase comparison circuit. The desired baseband modulation signal of the FM-modulated input signal is obtained in the DC component of the product of the two last-mentioned FM-modulated signals. This baseband modulation signal will be available at the output of this first phase comparison circuit and is applied to the output of the known FM quadrature demodulator after a selection in the low-pass filter.
However, as a result of the multiplication, higher order interference components are also produced in the known FM quadrature demodulator. Of these components, the frequency of the second-order interference component is closest to the desired DC component and generally has the largest amplitude with respect to the other interference components. An effective suppression of this second-order interference component can be obtained by making use of a low-pass filter having a higher-order slope. However, such a filter is difficult to integrate and costly. Moreover, such a filter realizes a comparatively large phase shift which considerably restricts the field of use of the known FM quadrature demodulator.